A liquid crystal display apparatus is provided with a backlight unit for irradiating a liquid crystal panel (LCD) with light from its rear side and illuminating the liquid crystal panel. The backlight unit comprises an illuminating light source, a light guide plate that causes light emitted from the light source to diffuse for irradiating the liquid crystal panel, a prism sheet or diffuser sheet by which the light radiated from the light guide plate is made uniform, and other such components.
The backlight unit in most of the current big-screen liquid crystal TV sets is of a so-called “direct illumination” type which has a light guide plate provided directly above an illuminating light source, as disclosed in Patent Document 1. In this type of backlight light, a plurality of cold-cathode tubes working as light sources are provided on a rear surface of a liquid crystal panel and the interior is made of white reflecting surfaces to ensure a uniform distribution of light quantity and the required luminance. However, this type of backlight unit, in principle, requires that the liquid crystal panel have a thickness of about 30 mm in a vertical direction in order to provide a uniform distribution of light quantity.
In recent years, liquid crystal display apparatuses are required to be thinner and consume less power while having a bigger screen; however, the effort toward smaller thickness has been limited in the above-described “direct illumination” type of backlight unit because unevenness in the quantity of light occurs if the thickness of the light guide plate is made smaller than 10 mm. In order to meet those demands of liquid crystal display apparatuses for smaller thickness, lower power consumption and bigger screen, light guide plates of various shapes have been proposed (see Patent Documents 2, 3, 4, 5, and 6).
FIG. 22 is an expanded perspective view that shows an outline of an area light source device having the light guide plate disclosed in Patent Document 2.
The area light source device (backlight unit) shown in FIG. 22 is formed by embedding fluorescent lamps 102 in a light guide plate 100, then providing a reflective sheet 104 on the back surface of the light guide plate 100 and placing a transmitted light quantity correcting sheet 106, a light diffuser 108, and a prism sheet 110 in superposition on an exit surface of the light guide plate 100.
The light guide plate 100 has a generally rectangular shape and is made of a resin into which fine particles that diffuse illuminating light are dispersed. Also, the light guide plate 100 has a flat upper surface, which is used as the exit surface. Further, grooves 100a each having a U sectional shape and into which the fluorescent lamps 102 are to be fitted are formed in the back surface (surface opposite to the exit surface) of the light guide plate 100, and a light quantity correcting surface 100b for promoting the emission of illuminating light is formed on the exit surface of the light guide plate 100, except in portions directly above the fluorescent lamps 102.
Thus, according to Patent Document 1, the fine particles are mixed into the resin to form the light guide plate 100 and in addition, the emission of illuminating light is promoted by the light quantity correcting surface 100b formed on a part or all of the exit surface except in the portions directly above the fluorescent lamps 102, thus making it possible to reduce the total thickness as well as the unnatural unevenness in luminance of the emitted light.
In addition, Patent Document 3 discloses a light guide plate which, for the purpose of obtaining a backlight for a liquid crystal display apparatus that is not reduced in the quantity of light it irradiates and which yet is capable of reducing the size, weight, thickness, cost and power consumption of the liquid crystal display apparatus, has a rectangular irradiating surface, a groove rectangular in section which is gouged out at a central portion with respect to the short sides in parallel with the long sides and in which a light source is to be inserted, and a back surface formed such that the plate thickness is gradually reduced from the groove toward both lateral surfaces on the long sides.
In addition, Patent Document 4 discloses a light guide member (light guide plate) which, for the purpose of obtaining a backlight unit that enables a liquid crystal display apparatus to have a narrower frame and a smaller thickness and which is bright with high utilization of light, has a recess into which a light source is to be provided and whose sectional shape as taken parallel to the direction of its width is a parabola having a major axis that extends in the direction of its depth.
Further in addition, Patent Document 5 discloses a light guide plate which, for the purpose of maintaining uniform brightness in a plane of a display panel to achieve illumination of high luminance, has a plurality of optical waveguide layers superposed on top of a generally V-shaped, highly reflective layer in such a way that their refractive indices increase sequentially to ensure that a light diffusing layer is made brighter by means of light emitting from the light exit surface of each optical waveguide layer. The recess in which a light source is to be provided has a triangular shape.
The light guide plates disclosed in the patent documents listed above are designed to fulfill some of the needs of liquid crystal display apparatuses, such as smaller thickness, miniaturization and weight reduction, lower power consumption, and lower cost; however, each of them is so adapted that one or more grooves are provided in the central part, with a rod of light source being fitted in each groove, with the sheet thickness being preferably rendered to decrease gradually from the groove toward an end face on either side, thereby successfully reducing the thickness of the liquid crystal display apparatus.
In addition, Patent Document 6 describes a liquid crystal backlight adapted for use on a big, liquid crystal display screen in a wall-hung TV set, in which a plurality of light guide plates are arranged side by side, with a predetermined number of linear light sources being provided such that each one is between adjacent guide plates, thereby providing high enough luminance to achieve highly uniform illumination of the back surface covering a wide area.
PATENT DOCUMENT 1: JU 5-4133 A
PATENT DOCUMENT 2: JP 9-304623 A
PATENT DOCUMENT 3: JP 8-62426 A
PATENT DOCUMENT 4: JP 10-133027 A
PATENT DOCUMENT 5: JP 5-249320 A
PATENT DOCUMENT 6: JP 2001-42327 A